Each letter means important thing for optimization of web page loading.
You should use link tag with attribute rel="preload" for the most important styles or scripts. Resources included this way will load first.
<link rel="preload" as="script" href="main.js">
In other words: it means: improve First Paint. It means to inline critical CSS and JS, and to add async attribute for other resources
<script src="main.js" async></script>
You could use Service Worker and Cache Storage API to cache resources at client.
Split your code into the chunks. Also use lazy loading for images.
<img src="example.jpg" loading="lazy" alt="" />
Inspired by: Apply instant loading with the PRPL pattern